Muscle contraction and many other forms of cell movement and changes in cell shape are believed to be driven by myosin molecules pulling themselves along actin filaments. Although the understanding of the ATP-dependent movement of myosin along actin is paramount to the understanding of cell motility, the molecular basis of this movement remains obscure. We have developed several in vitro assays for myosin movement on actin, two of which use purified actin and myosin. These assays allow us to quantitate the rate of movement of various forms of myosin. We are now in a position to determine which regions of the myosin molecule are required for motive force production and what aspects of those regions are important for movement. One overall goal of this proposal is to define in molecular terms the parameters of the moysin molecule necessary for its ability to move. We have succeeded in generating movement with short HMM a proteolytic fragment of mammalian muscle myosin missing the carboxy-terminal 110 nm of the 150 nm rod. Further work with proteolytic fragments of myosin will be carried out to determine, for example, whether two heads are necessary for short HMM to move. We will also use molecular genetic approaches to examine the movement capabilities of myosins altered by site-directed mutagenesis. Other experiments will be designed to determine the minimum number of myosin molecules needed for movement of a single actin filament. We also propose to examine whether organelle movements in vivo involve myosin- driven translocations analogous to the myosin-coated bead movements on actin filaments in vitro.